In recent years, in order to reduce CO2 emissions from the perspective of global environmental conservation, there has been a strong demand for weight reduction of automotive bodies. In addition, in order to ensure the safety of occupants in motor vehicle crashes, there has also been a strong demand for greater safety focusing on the crash characteristics of automotive bodies. To meet the demands, both weight reduction and reinforcement of automotive bodies must be achieved. It is therefore necessary to reduce the thickness of steel sheet serving as automotive body materials without significantly reducing the rigidity of the steel sheet. Furthermore, in order to improve crash resistance, it is also necessary to increase the yield ratio (YR) or the bake hardenability (BH) of the steel sheet.
Patent Literature 1 discloses a steel sheet containing C: 0.04% to 0.15%, Si: 0.20% or less, Mn: 1.0% to 2.5%, P: 0.050% or less, S: 0.020% or less, Al: 0.010% to 0.120%, and Cr: 0.1% to 2.0% and having a complex structure of three phases: ferrite, martensite, and bainite. This steel sheet has a high ductility and a high BH characteristic of a ferrite and martensite complex structure, and the additional bainite structure decreases hard martensite, which is the starting point of voids and reduces stretch flange formability.
Patent Literature 2 discloses a steel sheet containing C: 0.04% to 0.22%, Si: 1.0% or less, Mn: 3.0% or less, P: 0.05% or less, S: 0.01% or less, Al: 0.01% to 0.1%, N: 0.001% to 0.005%, and at least one selected from Nb, Ti, and V: 0.008% to 0.05% in total. The steel sheet has a ferrite and martensite complex structure. The martensite has a maximum particle size of ≦2 μm and an area percentage of ≧5%. This structure can have a reduced number of starting points of voids, which affect stretch flange formability, and thereby improve stretch flange formability. The martensite having an area percentage of ≧5% provides a high BH to the steel sheet.
However, in the technique described in Patent Literature 1, the BH is not more than 51 MPa, and the YR is as low as 0.51 to 0.58. Thus, it is necessary to further improve crash resistance. With respect to the technique described in Patent Literature 2, in spite of the high BH and excellent crashworthiness, there is no description of indicators of press formability, such as ductility and uniformity of mechanical characteristics.
In general, a high-strength steel sheet having a tensile strength (TS) of 590 MPa or more contains large amounts of various alloying elements for reinforcement. Thus, a variation in manufacturing conditions may cause a variation in type and amount of precipitates or second phases and result in a significant variation in mechanical characteristics, such as strength or elongation, in a coil of the steel sheet, particularly in the longitudinal direction of the coil. Such a variation makes it difficult to perform stable press forming in a continuous press line for automobiles and significantly reduces workability. Thus, the excellent uniformity of mechanical characteristics in a coil is strongly requested.
There have been many propositions of a technique for improving the uniformity of mechanical characteristics in a coil of high-strength steel sheet. For example, Patent Literature 3 discloses a technique for homogenizing the mechanical characteristics in a coil by adding Ti and Nb to a steel having a low carbon content of 0.0070% or less and coiling at a coiling temperature of 620° C. or more after hot-rolling the steel. In this technique, N responsible for a variation in mechanical characteristics is precipitated as TiN rather than AlN before finish rolling, and C is precipitated as complex carbides (Ti, Nb)C. However, the coiling temperature may decrease to less than 620° C. or locally decrease to less than 620° C. in actual operation, and this can cause a variation in precipitation behavior in the coil, resulting in a significant variation in mechanical characteristics. In particular, a low atomic ratio of Ti and Nb to C results in an insufficient precipitation of carbides and a significant degradation of mechanical characteristics at a top or bottom portion of the coil that can be relatively easily cooled.
Patent Literature 4 discloses a technique for reducing the dependence of mechanical characteristics, such as strength and elongation, on the coiling temperature by controlling the C content to more than 0.0050% and 0.010% or less and (Nb %×12)/(C %×93)=1.6 to 2.4. However, this technique is applicable to single-phase ferritic steel based on interstitial free (IF) steel, which is ultra-low carbon steel, and high-strength steel sheets having a tensile strength of 590 MPa or more are not described.